The ubiquitous integrated chips utilized in the making of modern electronic devices are constructed from small, fragile silicon wafers. It is imperative that these wafers possess flat, blemish-free, mirror-like surfaces, since surface imperfections can adversely affect the electrical properties of the final integrated chips.
Typically, substrate wafers are cut by diamond-sawing single crystal silicon rods. In order to ensure integrated chips of sound structural integrity, it is first necessary to remove the toughened portion of the crystal surface which was produced during the sawing procedure.
The first operation for obtaining a blemish-free surface is called "lapping". The lapping operation employs a coarse abrasive such as coarse alumina or silicon carbide abrasive particles. Lapping removes coarse surface imperfections from the sawing operation. Lapping also provides flatness and parallelism to the surface.
After the lapping operation, a series of polishing steps are employed to eliminate the remaining surface imperfections.
During the polishing operation, a number of silicon wafers are typically mounted or "fixtured" onto a metal or ceramic carrier or polishing head by a template assembly or by an adhesive material, in order to eliminate the need for manually polishing each individual wafer. The template assembly consists of impregnated polyurethane and plastic retaining rings to hold the silicon wafers in place. Adhesive materials may be a wax or a rosin dissolved in a chlorinated hydrocarbon solvent.
Unfortunately, certain difficulties arise in the fixturing procedure, whether the silicon wafers are attached by a template assembly or mounted using an adhesive material. For example, when the silicon wafers are attached onto the metal or ceramic carrier or polishing head by a template assembly, the polyurethane layer compresses, thereby allowing the wafer to move as it is being polished. This movement can result in uneven polishing, rounded edges and a loss of flatness.
Alternatively, when the adhesive mounting method of silicon wafers is employed, air bubbles can become entrapped in the adhesive layer. If the surface of the wafer comes into contact with the sticky adhesive layer soon after it is applied to the carrier or polishing head and before all of the air is forced out from beneath the wafer, some air bubbles remain trapped between the wafer and surface of the carrier or polishing head. The desired flatness of the silicon wafer surface cannot be achieved when such air bubbles are entrapped and allowed to remain during polishing.
Today, with the advent of very large scale integration (VLSI) chips, it has become more critical than ever to achieve an extremely flat surface. The especially flat blemish-free surfaces requiring the multiple layers of circuitry found in VLSI chips cannot be achieved if air bubbles are trapped below the wafer in the fixturing step or if the silicon wafer is able to move as it is being polished.
It has been suggested that these problems can be reduced or eliminated by spin coating an adhesive film onto a metal or ceramic carrier. U.S. Pat. No. 3,475,867 to Robert J. Walsh discloses the application of an adhesive wax solution onto a rotating carrier plate. The plate is rotated at a sufficient speed to cause the wax to spread out uniformly over the entire surface of the carrier plate. The wax is then heated, yielding a tacky surface to which the wafers are applied.
Alternatively, U.S. Pat. No. 4,316,757 to Robert J. Walsh discloses the application of a wax solution onto a highly machined alumina carrier followed by spinning to spread the coating. The carrier is subsequently heated from below to cure the adhesive and drive off the solvent. When the adhesive becomes tacky, the silicon wafer is mounted under vacuum to prevent the entrapment of air bubbles. Once the carrier is cooled and polished, the wafers are demounted and cleaned.
Unfortunately, both of the above methods give rise to the formation of static charge during the spinning step.
A charged stainless steel or mild steel ring may be used to dissipate the static charges which develop on the surface of the metal or ceramic carriers. However, current must be supplied to the ring in this process, making this a costly solution. Furthermore, additional static charges may still be induced into the adhesive coating. Unless the environment is extremely clean, dirt particles from the air will be attracted and absorbed into the charged adhesive film. The adhesive film will deform around the particulate matter, thereby causing imperfections or "dimples" on the polished surface of the wafer. The resulting defects and non-uniform wafer surface can disrupt electronic flow and cause electrical problems. Moreover, induced charge on the wafer may attract oppositely charged undesirable dust or dopant particles.
In addition to the above requirements, fixturing adhesives should not contain materials which even in small quantities would act as semi-conductor dopants. Materials containing boron, phosphorous and metals should be avoided.
Solvents utilized in the manufacture of antistat adhesives should have a high enough flash point to readily evaporate under vacuum with only minimal heating and should not cause unwanted flow patterns of the adhesive during drying. Unwanted flow patterns of the adhesive/solvent mixture can cause pooling at some locations, and solvent depletion in others. This forming of uneven surfaces is called "bumps" and "sinks" defects.
Therefore, it would be highly desirable to develop a practical, economically attractive method for reducing or eliminating static charge on fixturing adhesive films, to prevent the attraction of airborne particulates to the adhesive film and wafer surfaces, thereby maintaining the quality of the adhesive film coating and the subsequent quality of any polished wafers mounted during polishing with this fixturing adhesive.